I tried to measure the magnification of the 18 mm - 200 mm with the Canon 500D close-up lens and my estimate is that it is approximately 1:2.5
I used a quarter and set the lens to 200 mm.. The diameter of the quarter is 24 mm which is very close to the width of the sensor. Using the rulers in Photoshop it appears that the quarter will fit 2.5 times across the width of the frame. I got this method from this web site.
An interesting aspect of this is that the magnification appears greatest at the point where the front of the lens is furthest from the subject and can still focus. I think this is because the 18 - 200 is really 200 mm only at infinity and as you move closer the focal length is reduced.